The noise generated by working machinery is commonly referred to as the acoustic signature of the machine. Noise often represents wasted useful work that can adversely affect overall machine efficiency. This is especially true of turbomachines, such as steam turbines, where noise is indicative of fluid energy that is not directed into the shaft of the turbomachine, but is instead wasted as fluid noise energy that decreases efficiency. The acoustic signature of a turbomachine can emanate from several fluid dynamic sources, such as, wake cutting, high velocity fluid dynamics, and turbulent flow fields. In order to increase the overall efficiency of the turbomachine, there is a continued effort to discover new and improved ways to direct wasted fluid noise energy to the shaft where it can produce useful work.
During operation of a steam turbine at full load, steam is admitted to the first stage, or “control stage,” through a first set of nozzle vanes arranged in a diaphragm. The diaphragm defines a large circumferential arc disposed upstream of the rotor blades of the first stage. In many steam turbines, the diaphragm is divided into a series of “partial arcs” into which the steam is admitted by means of individual throttle valves. The partial arcs are commonly called the “arcs of admission” of the steam turbine. For operation at low or part load, a given arc of admission may be relatively small, for example, a quarter of the full circumferential arc of the diaphragm, or sometimes even less. This segmentation of the diaphragm allows the steam velocity past the nozzle vanes to be equivalent to that at full load operation, for which the rotor blades are specifically designed and where high turbine power and efficiency are critical. Consequently, steam turbine efficiency may be improved at low and intermediate loads.
As each rotor blade enters and exits the arc of admission during low load operation, however, it is subjected to sudden and immediate load impulses created by the working fluid. These load impulses are absorbed by each passing blade and can generate inefficiencies in the form of undesirable noise, such as frequencies at the harmonics of the nozzle passing frequency. Moreover, the load impulses impart bending forces on each blade which can contribute to the fatigue of the blade material and thereby reduce rotor blade life. As a result, rotor blades are often required to be over-designed to make them more robust and therefore strong enough to endure for the useful life of the rotor assembly.
What is needed, therefore, is a method and system configured to reduce or otherwise mitigate the sudden load impulse absorbed by rotor blades as they enter and exit the arc of admission in a steam turbine operating at low load.